This study phase will be concerned with investigations of the interaction of fluoride and magnesium in man. Fluoride and magnesium balances will be determined under strictly controlled dietary conditions in control studies during a low fluoride and low magnesium intake and during supplementation of the diet with fluoride alone, with magnesium 0lone, and with both fluoride and magnesium. The low fluoride intake will be due to the fluoride content of the constant diet and due to the intake of fluoridated water. The fluoride supplements will be given as sodium fluoride. The low magnesium intake will be that contained in the metabolic diet. The magnesium supplement will be given as magnesium oxide thereby increasing the magnesium intake about 3-fold. As the amount of calcium and of phosphorus in the diet may affect the metabolism of both fluoride and of magnesium, the studies will be carried out during three different intakes of calcium and two intake levels of phosphorus will be used during each intake level of calcium. As fluoride as well as magnesium in the intestinal tract may affect the intestinal absorption of calcium this parameter will be investigated in each study phase by using oral tracer doses of Ca 47.